1. Field of the Invention
The present invention relates to an optical image measuring apparatus that applies a light beam to an object to be measured, particularly a light scattering medium, and produces a surface form or inner form of the object to be measured by detecting a reflected light beam or a transmitted light beam. In particular, the present invention relates to an optical image measuring apparatus for measuring the surface form or inner form of the object to be measured by using an optical heterodyne detection method to produce the image of the measured form.
2. Description of the Related Art
In recent years, attention has been given to optical imaging technique that produces an image of a surface or inner portion of an object to be measured using a laser light source or the like. This optical imaging technique is not hazardous to human bodies in contrast to the conventional X-ray CT. Therefore, the development of applications in the medical field has been particularly expected.
An example of a typical method of the optical imaging technique is a low coherent interference method (also called ‘optical coherence tomography’ or the like). This method uses the low coherence of a broad-band light source having a broad spectral width, such as a super luminescent diode (SLD). According to this method, reflection light from an object to be measured or light transmitted therethrough can be detected at superior distance resolution on the order of μm (for example, see Naohiro Tanno, “Kogaku” (Japanese Journal of Optics), Volume 28, No. 3, 116 (1999)).
FIG. 5 shows a basic structure of a conventional optical image measuring apparatus based on a Michelson interferometer, as an example of an apparatus using the low coherent interference method. An optical image measuring apparatus 100 includes a broad-band light source 101, a mirror 102, a beam splitter 103, and a photo detector 104. An object to be measured 105 is made of a scattering medium. A light beam from the broad-band light source 101 is divided by the beam splitter 103 into two, that is, reference light R propagating to the mirror 102 and signal light S propagating to the object to be measured 105. The reference light R is light reflected by the beam splitter 103. The signal light S is light transmitted through the beam splitter 103.
Here, as shown in FIG. 5, a propagating direction of the signal light S is set as a z-axis direction and a plane orthogonal to the propagating direction of the signal light S is defined as an x-y plane. The mirror 102 is movable in a direction indicated by a double-headed arrow in FIG. 5 (z-scanning direction).
The reference light R is subjected to a Doppler frequency shift through when reflected by the z-scanning mirror 102. On the other hand, the signal light S is reflected from the surface of the object to be measured 105 and from the inner layers thereof when the object to be measured 105 is irradiated with the signal light S. The object to be measured 105 is made of the scattering medium, so reflection light of the signal light S may be a diffusing wave having random phases. The signal light propagating through the object to be measured 105 and the reference light that propagates through the mirror 102 to be subjected to the frequency shift are superimposed on each other by the beam splitter 103 to produce interference light.
In the image measurement using such a low coherent interference method, interference occurs only when a difference in optical path length between the signal light S and the reference light R is within the coherence length (coherent distance) on the order of μm of the light source. In addition, only the component of the signal light S whose phase is correlated to that of the reference light R interferes with the reference light R. That is, only the coherent signal light component of the signal light S selectively interferes with the reference light R. Based on their principles, the position of the mirror 102 is shifted by the z-scanning to vary the optical path length of the reference light R, so that a reflectance profile of the inner layers of the object to be measured 105 is measured. The object to be measured 105 is also scanned with the irradiated signal light S in an x-y plane direction. The interference light is detected by the photo detector 104 during such scanning in the z-direction and the x-y plane direction. An electrical signal (heterodyne signal) outputted as a detection result is analyzed to obtain a two-dimensional sectional image of the object to be measured 105 (see Naohiro Tanno, “Kogaku” (Japanese Journal of Optics), Volume 28, No. 3, 116 (1999)).
Assume that an intensity of the reference light R and an intensity of the signal light S which are superimposed by the beam splitter 103 are given by Ir and Is, respectively, and a frequency difference between the reference light R and the signal light S and a phase difference therebetween are given by fif and Δθ, respectively. In this case, a heterodyne signal as expressed by the following expression is outputted from the photo detector (for example, see Yoshizawa and Seta “Optical Heterodyne Technology (revised edition)”, New Technology Communications (2003), p. 2).
                    Expression        ⁢                                  ⁢                  (          1          )                                                                              i          ⁡                      (            t            )                          ∝                              I            r                    +                      I            s                    +                      2            ⁢                                                            I                  r                                ⁢                                  I                  s                                                      ⁢                          cos              ⁡                              (                                                      2                    ⁢                                                                                  ⁢                    π                    ⁢                                                                                  ⁢                                          f                      if                                        ⁢                    t                                    +                                      Δ                    ⁢                                                                                  ⁢                    θ                                                  )                                                                        (        1        )            
The third term of the right side of the expression (1) indicates an alternating current electrical signal and the frequency fif thereof is equal to the frequency of beat caused from the interference between the reference light R and the signal light S. The frequency fif of an alternating current component of the heterodyne signal is called a beat frequency or the like. The first and second terms of the right side of the expression (1) indicate the direct current components of the heterodyne signal and correspond to a signal intensity of background light of interference light.
However, when the two-dimensional cross sectional image is obtained by the conventional low coherent interference method, it is necessary to scan the object to be measured 105 with a light beam and to successively detect reflection light waves from respective regions of the object to be measured 105 in a depth direction (z-direction) and a sectional direction (x-y plane direction). Therefore, the measurement of the object to be measured 105 requires a long time. In addition, it is hard to shorten a measurement time in view of measurement fundamentals.
In views of such problems, an optical image measuring apparatus for shortening a measurement time has been proposed. FIG. 6 shows a fundamental structure of an example of such an apparatus. As shown in FIG. 6, an optical image measuring apparatus 200 includes a broadband light source 201, a mirror 202, a beam splitter 203, a two-dimensional photo sensor array 204 serving as a photo detector, and lenses 206 and 207. A light beam emitted from the light source 201 is converted into a parallel light flux by the lenses 206 and 207 and a beam diameter thereof is widened thereby. Then, the parallel light flux is divided into two, that is, the reference light R and the signal light S by the beam splitter 203. The reference light R is subjected to Doppler frequency shift through z-scanning with the mirror 202. On the other hand, the signal light S is incident on the object to be measured 205 over a broad area of the x-y plane because the beam diameter is widened. Therefore, the signal light S becomes reflection light including information related to the surface and inner portion of the object to be measured 205 over a wide area. The reference light R and the signal light S are superimposed on each other by the beam splitter 203 and detected by elements (photo sensors) arranged in parallel on the two-dimensional photo sensor array 204. Thus, it is possible to obtain a two-dimensional cross sectional image of the object to be measured 205 in real time without light beam scanning.
An apparatus described in K. P. Chan, M. Yamada, and H. Inaba, “Electronics Letters”, Vol. 30, 1753 (1994) has been known as such a non-scanning type optical image measuring apparatus. In the apparatus described in the same document, a plurality of heterodyne signals outputted from a two-dimensional photo sensor array are inputted to signal processing systems arranged in parallel to detect the amplitude and phase of each of the heterodyne signals.
However, when the spatial resolution of an image is increased, it is necessary to increase a number of elements of the array. In addition, it is necessary to prepare a signal processing system including a number of channels corresponding to the number of elements. Therefore, it is supposedly hard to actually use the apparatus in fields that require a high resolution image, such as a medical field and an industrial field.
Thus, the inventors of the present invention have proposed the following non-scanning type optical image measuring apparatus in JP 2001-330558 A (claims and specification paragraphs [0044] and [0072] to [0077]). The optical image measuring apparatus according to this proposal includes a light source for emitting a light beam, an interference optical system, and a signal processing portion. In the interference optical system, the light beam emitted from the light source is divided into two, that is, signal light propagating through an examined object arrangement position in which an object to be examined is arranged and reference light propagating on an optical path different from an optical path passing through the examined object arrangement position. The signal light propagating through the examined object arrangement position and the reference light propagating on the different optical path are superimposed on each other to produce interference light. The interference optical system includes a frequency shifter, light cutoff devices, and photo sensors. The frequency shifter shifts a frequency of the signal light and a frequency of the reference light relative to each other. In order to receive the interference light in the interference optical system, the interference light is divided into two parts. The light cutoff devices periodically cut off the two divided parts of the interference light to generate two interference light pulse trains with a phase difference of 90 degrees therebetween. The photo sensors respectively receive the two interference light pulse trains. The photo sensors each have a plurality of light receiving elements which are spatially arranged and separately detect light receiving signals. The signal processing portion combines the plurality of light receiving signals detected by the photo sensors to generate signals of the signal light which correspond to respective points of interest of a surface or inner layers of the object to be examined which is arranged in the examined object arrangement position on a propagating path of the signal light.
In the optical image measuring apparatus, the interference light in which the reference light and the signal light interfere with each other is divided into two parts. The two parts of the interference light are received by the two photo sensors (two-dimensional photo sensor arrays) and respectively sampled by the light cutoff devices (shutters) disposed in front of both sensor arrays. A phase difference of π/2 is set between sampling periods of the two divided parts of the interference light. Therefore, an intensity of the signal light and an intensity of the reference light which compose background light of the interference light and phase quadrature components (sine component and cosine component) of the interference light are detected. In addition, an intensity of the background light included in outputs from both the sensor arrays is subtracted from the outputs of both the sensor arrays to calculate two phase quadrature components of the interference light. An amplitude of the interference light is obtained based on the calculation result.
However, according to the optical image measuring apparatus described in JP 2001-330558 A (claims, specification paragraphs [0068] to [0084], and FIG. 1), it is necessary that the interference light be divided into a plurality of optical paths, the light cutoff device be provided on each of the optical paths, and separate interference light beams be sampled while the respective light cutoff devices are synchronized with one another. Therefore, an apparatus structure and control are complicated and it is likely to increase a cost. When a transmission type shutter such as a liquid crystal shutter is used as the light cutoff device, a signal loss of each of the interference light beams may be increased to reduce the detection sensitivity of each of the interference light beams, so it is likely to deteriorate the sensitivity of measurement.
An available image sensor such as a charge-coupled device (CCD) camera has been widely used for the two-dimensional photo sensor array of the optical image measuring apparatus as described above. However, up to now, a problem has been recognized that a currently available CCD camera cannot follow the beat frequency of a heterodyne signal which is on the order of several kHz to several MHz because of the low frequency response characteristic thereof. The feature of the optical image measuring apparatus which is proposed by the inventors of the present invention and described in JP 2001-330558 A is to perform the measurement using the low frequency response characteristic based on the sufficient understanding of the problem.